The electroreflectance (ER) effect has been known for many years, and occurs in many materials. The ER effect describes the fact that the optical reflectivity of a material changes when a strong electric field is applied to the material. For each such material, there are specific wavelengths of light, or ranges of wavelengths, at which the ER effect exists.
An electric field may be applied to an electroreflective material by a variety of techniques. As illustrated in FIG. 1, electrodes 12 and 14 may be attached to electroreflective material 16 and to a voltage source, such that an electric field designated by electric field line 18 is produced in material 16. The area 20 between the electrodes is then illuminated with a collimated and monochromatic light beam 22, and the intensity of the reflected signal may be measured by a suitable photodetector. Variations in the intensity of the reflected signal that are correlated to voltage variations between electrodes 12 and 14 provide a measure of the ER effect. In this arrangement, the electric field is parallel to the reflecting surface at area 20. In general, the ER effect is observed for all orientations of the electric field with respect to the direction of propagation and the polarization of the incident light. The term "light" is used herein in a conventional sense to include infrared, visible and ultraviolet radiation, and the term "optical" is used to refer to such light.
FIG. 2 illustrates an alternate technique in which electroreflective material 30 is sandwiched between front electrode 32 and back electrode 34. Front electrode 32 is at least partially transparent. When a potential is applied between electrodes 32 and 34, an electric field designated by electric field line 36 is created in electroreflective material 30. Light beam 40 is directed onto transparent front electrode 32, and passes through the front electrode and onto surface 38 of electroreflective material 30, where a portion of the light is reflected. Variations of this arrangement include those in which a transparent insulator is placed between front electrode 32 and electroreflective material 30, and those in which front electrode 32 is replaced by an electrolyte, such as weak solution of potassium chloride in water. In any of these arrangements, one can observe that light in the appropriate wavelength ranges for which the ER effect occurs and, upon reflection from the electroreflective material, is no longer of constant intensity, but rather includes a small component whose magnitude depends on the magnitude of the applied electric field.
Unfortunately, the dependence of the reflectivity on electric field for known electroreflective materials is very small. Even for a high electric field on the order of 50,000 volts/cm. one is likely to see a change in reflectivity of only one part in several hundred, or perhaps even less. For this reason, the ER effect has not been exploited for the measurement of electric field strength.